The past decade has witnessed the discovery of thirty or more peptides localized in mammalian central neurons. Cholecystokinin (CCK) in a gut peptide recently found in microgram concentrations in cerebral cortex, limbic structures, and spinal cord. Cholecystokinin appears to coexist with dopamine in the mesolimbic pathway from ventral tegmentum to nucleus accumbens and olfactory tubercle. Behavioral techniques provide useful tools for elucidating the mechanism of neuromodulation of a known neurotransmitter by a peptide. Understanding of CCK modulation of dopamine may lead to the development of more specific and effective antipsychotic treatments.